This invention generally relates to chemically amplified photoresist compositions and processes for its use. More particularly, the photoresist composition includes a blend of a phenolic polymer and a (meth)acrylate based copolymer.
There is a desire in the industry for higher circuit density in microelectronic devices which are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. The use of shorter wavelength radiation (e.g., 190 to 315 nm) offers the potential for higher resolution. However, with deep UV radiation, fewer photons are transferred for the same energy dose and higher exposure doses are required to achieve the same desired photochemical response. Further, current lithographic tools have greatly attenuated output in the deep UV spectral region.
In order to improve sensitivity, several acid catalyzed chemically amplified photoresist compositions have been developed such as those disclosed in U.S. Pat. No. 4,491,628 and Nalamasu et al., “An Overview of Resist Processing for Deep-UV Lithography”, J. Photopolymer Sci. Technol. 4, 299 (1991). The photoresist compositions generally comprise a photosensitive acid generator (i.e., photoacid generator or PAG) and an acid sensitive polymer. The polymer has acid sensitive side chain (pendant) groups which are bonded to the polymer backbone and are reactive towards a proton. Upon imagewise exposure to radiation the photoacid generator produces a proton. The photoresist film is heated and, the proton causes catalytic cleavage of the pendant group from the polymer backbone. The proton is not consumed in the cleavage reaction and catalyzes additional cleavage reactions, thereby chemically amplifying the photochemical response of the resist. The cleaved polymer is soluble in polar developers such as alcohol and aqueous base while the unexposed polymer is soluble in nonpolar organic solvents such as anisole. Thus, the photoresist can produce positive or negative images of the mask depending on the selection of the developer solvent. Advances in resist technology have provided chemically amplified photoresist compositions capable of resolving sub-22 nm lines using 193 nm immersion techniques. While these high performance materials are essential for stringent front end design requirements, they are generally unsuitable for back end applications due to their extremely high cost, complex processing conditions, and relatively thin film restrictions. Thus, for back end requirements, less sophisticated resists are used such as those described by Ito, “Chemical Amplification Resists for Microlithography”, Adv. Polym. Sci. (2005) 172, 37-245. These resists, capable of resolving features down to the range of about 100-130 nm, generally comprise a protected (acid sensitive protecting group) hydroxystyrene/acrylate copolymer and operate at an incident wavelength of 248 nm. While these photoresists are currently firmly established in the industry, they do suffer from relatively high production costs. In order to address this issue Allen et al. disclosed in U.S. Pat. No. 5,372,912 a blend of commercially available poly(meth)acrylate and phenolic polymers (e.g., novolac resins), which retained the advantages of the hydroxystyrene/acrylate copolymer commonly used in the front end applications (at that time) at a significantly lower cost. There still remains, however, a need in the industry for a low cost resist with improved resolution and the flexibility to encompass thick film applications (such as liquid crystal and other flat panel displays) and various dissolution and etch resistance requirements.
Although chemically-amplified resists have been developed for 248, 193 and 157 nm lithography, certain barriers to achieving higher resolution and smaller feature sizes remain due to physical, processing and material limitations. One such phenomenon that arises, resulting in diminished image integrity in the pattern, is referred to as “image blur” (see, e.g., Hinsberg et al., Proc. SPIE, (2000), 3999, 148 and Houle et al., J. Vac. Sci. Technol B, (2000), 18, 1874). Image blur is generally thought to result from two contributing factors: gradient-driven acid diffusion (acid mobility) and reaction propagation, the result being a distortion in the developable image compared to the projected aerial image transferred onto the film. The acid mobility in the polymer is dependent on a variety of factors, including, among others, the chemical functionality of the polymer; the temperature and time of baking during resist processing and the presence of acid-quenchers. The extent of thermally induced image blur is estimated to be on the order of 50 nm with conventional resists and processing (see also Breyta et al., U.S. Pat. No. 6,227,546). R. Sooriyakumaran recently disclosed two low blur technologies for 193 nm FARM resists, the first of which is based on various acid-labile acetal/ketal protecting groups (U.S. Pat. No. 7,193,023, Filed Dec. 4, 2003) with surprisingly low processing (PEB) temperatures in the presence of stoichiometric water molecules and the second of which (U.S. Pat. No. 7,476,492, Filed May 26, 2006) involves acid-labile tertiary alkyl esters also with unexpectedly low PEB temperatures. The latter technique benefits from longer shelf life and less run-to-run variability and does not require adventitious moisture to operate. The exceptionally low PEB temperatures realized by these resists is largely the result of acidic comonomers (i.e., hexafluoroalcohols and fluorosulfonamides) which provide a mildly acidic bulk environment thereby abetting the action of photogenerated acid. The major disadvantage of these resists is their high cost of manufacture and their relatively high absorbance at 193 nm thus restricting their use to very thin films in front end applications.
Accordingly, there is a need in the art for improved radiation sensitive photoresist compositions for back end applications that are low cost, capable of resolving feature sizes of less than 100 nanometers (nm), and are of lower absorbance so as to permit the use of these photoresists in thicker film applications.